The present invention relates to a method for making holes in ceramic green sheets and to a device for carrying out the method.
Multilayer ceramic substrates, such as LTCC (low-temperature cofired ceramic) substrates, are known to be made from a stack of xe2x80x9cceramic green sheets.xe2x80x9d The term xe2x80x9cceramic green sheetsxe2x80x9d denotes unfired ceramic sheets in the raw state, which are pliable and ductile within set limits. Before being stacked, the ceramic green sheets are provided with holes and are then printed with conductor paste by the screen printing process to produce conductive lands. The conductor paste at the same time makes its way into the holes, thereby producing electrical feedthroughs known as vias, which later will interconnect the various circuit layers of the finished multilayer substrate. Once printed, the ceramic sheets are dried, stacked, laminated and fired.
The holes in the ceramic green sheets are usually made by punching. Such a method is known, for example, from Japanese Laid Open Patent Application No. 11058297. For this purpose, the ceramic green sheet to be processed is laid on a support plate provided with apertures. A punch is then used in the conventional manner to punch holes in the ceramic green sheet at the prescribed locations. The punch is made of tungsten, for example. The material punched out at the hole locations is conveyed as chads from the tool tip of the punch through the apertures in the support plate into a receiving space, where it is blown away from the tip of the punch by compressed air. A disadvantage of the known method is that the punches are subjected to high mechanical stresses and thus wear out quickly. This disadvantage is exacerbated by the fact that often very small holes with a circular diameter of, for example, 0.5 mm are punched in the ceramic green sheet. Breakage of punches during such operations is not uncommon. A further disadvantage results from the fact that despite removal of the punchings by compressed air, which requires substantial outlay, it is not always possible to ensure that the portion of the punch tip acting on the ceramic sheets is free of chads. These often adhere to the tip and are difficult to remove. As a result, there is always the possibility of damage to the ceramic sheet and deviation of the shape of the holes from what is intended.
The method according to the present invention avoids the known disadvantages of the related art. The fact that a punch is no longer used to make the holes saves the set-up time and cost involved in replacing a damaged punch. According to the present invention, the holes are made by using a gas placed under a higher pressure than atmospheric pressure, which is expelled in a pulsed manner from a pressure vessel and thereby acts on the ceramic green sheet, at at least one location where a hole is to be made, in such a way that the expelled gas shoots material out of the ceramic green sheet at that location. Material fatigue and wear phenomena of the kind that occur with the use of a punch can be avoided in this way. There is no possibility of damage to the ceramic green sheets and the support plate due to breakage of a punch. In addition, it is particularly advantageous that there is no need for the lateral blowing-off and cleaning of the punch tip. For the reasons cited, the cost of producing multilayer ceramic substrates can be greatly reduced.